The increasing incidence of melanoma and mortality associated with advanced stages of the disease are cause for concern. Recent reports on the efficacy of therapeutic agents targeting the most common alteration in melanoma, mutant BRAF, have been very encouraging and have shifted the treatment paradigm for this disease. Despite these impressive clinical successes, a significant percentage of patients are intrinsically resistant to BRAF inhibitors and those who initially respond ultimately relapse as a result of acquired resistance. In addition, nearly half of all melanomas contain wild-type BRAF and are therefore not sensitive to mutant BRAF inhibitors. While targeted therapies have the potential to revolutionize cancer care by providing personalized treatment strategies that are less toxic and more effective, maximizing their effectiveness requires increased knowledge of the molecular alterations that drive tumor formation, progression, maintenance, and resistance. In vivo models fulfill this need. We have developed a novel mouse model of melanoma that not only aids in the identification and validation of alterations that drive tumor growth but also provides a means to assess tumor progression and efficacy of therapeutic strategies. Importantly, melanomas can be induced in our model using the same genetic alterations observed in the human disease and specific gene combinations result in both lung and brain metastases, which are responsible for nearly half of all melanoma patient deaths. We have observed that despite their high sequence and functional similarity, AKT2 and AKT3 significantly differ in their ability to promote brain metastases in the context of mutant BRAF in vivo. Interestingly, brain metastases are also not observed in mice with mutant BRAF-driven tumors that lack Pten, which is believed to contribute to melanoma development and progression through deregulated AKT activity. We are uniquely poised to investigate the mechanistic differences between these tumors in vivo using our novel mouse model. We hypothesize that the differential ability of the three AKT isoforms and Pten loss to induce brain metastases is due to differential downstream signaling and that genetic suppression of AKT3 in this context will significantly delay the development and growth of distant metastases. To test this hypothesis, we will define the functional domain(s) that dictate AKT isoform specific melanoma brain metastases in vivo, use functional proteomics to identify AKT3-dependent effectors involved in promoting brain metastases, and utilize an in vivo genetic approach to evaluate the effect of targeting AKT3 on melanoma growth and metastasis. We will further validate our findings in melanoma tissue from patients. The long-term goals of this project are to identify targets that promote brain metastases as well as biomarkers that identify the subset of melanoma patients that are most likely to develop brain metastases and would therefore benefit from more aggressive therapy.